OSI Layer 1 defines physical links. There are quite a few types of attacks that can be found at Layer 1, including:
Cable / Fiber cuts
High voltage applied on copper lines
Wireless links jamming
Electromagnetic field sources brought near copper cables, etc.
Securing the physical layer is beyond the scope of this book and must be done by field or transmission engineers.
However, it is important for network administrators to know how the physical links are built, and to have backup routes for the most important ones as well as for the most exposed ones.